9200T going for landfill

In article , Mike Tomlinson wrote:
I've just looked at an external SCSI DLT tape drive. That had bad caps
on the 12V output of the PSU, causing it to rise to 20V under load. No
overvoltage protection was fitted.

Can you explain how a faulty capacitor makes a voltage rise?

Rod.
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In article , Mike Tomlinson wrote:
I'm astonished by the number of electronic faults that are diagnosed remotely
as requiring nothing more than replacement of all the capacitors,

I have 25 years of experience, so do have an idea of what I speak.

I have about 45 years of experience. Please enlighten me.

Rod.
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Roderick Stewart wrote:
In article , Mike Tomlinson wrote:
I've just looked at an external SCSI DLT tape drive. That had bad
caps on the 12V output of the PSU, causing it to rise to 20V under
load. No overvoltage protection was fitted.

Can you explain how a faulty capacitor makes a voltage rise?

Rod.

It loses capacitance, resulting in an increased HF ripple across it, but the
feedback signal is HF filtered before going to the comparator,
so the output is regulated to nearer the mean value rather than the peak.

Martin.

"Roderick Stewart" wrote in
message .myzen.co.uk...

I have 25 years of experience, so do have an idea of what I speak.

I have about 45 years of experience. Please enlighten me.

Oh, for goodness' sake, let's stop the willy-waving, shall we?

I've known - from my old job - people with 45 years' experience who were
incompetent *******, and I've known people with 5 years experience to be
scarily good. And, of course, vice versa.

Experience is usually valuable, but frankly I simply don't buy the idea that
45 years is any better than 25 years, when we're talking about servicing a
surface-mount PCB. I'm not convinced, either, that 25 years is any better
than one year, if that year has been spent on the bench with a soldering
iron for 40 hours per week.

This debate - whether swapping out the power supply capacitors does more
harm than good - is complete nonsense because neither side has presented any
objective evidence to support their claim. We are simply arguing opinions
and assertions.

Let's see some peer-reviewed, published studies on PCB repair policies and
practices - so at least we've got some facts - before flogging this one any
more, shall we?

SteveT

In article , Steve Thackery wrote:
This debate - whether swapping out the power supply capacitors does more
harm than good - is complete nonsense because neither side has presented any
objective evidence to support their claim. We are simply arguing opinions
and assertions.

My assertion was concerned with swapping components unnecessarily without doing
any diagnostics to establish which one is actually faulty. In my experience,
any interference with things that are not faulty is a bad thing and can lead to
further problems that were not originally present, particularly when we are
talking about small components on crowded circuit boards. Tracks, and nearby
components, can become damaged by excessive heat from a soldering iron, and
there may be complications if it is not possible to obtain exact replacements
for the components that are removed, because the repaired equipment effectively
has a different circuit from the original, or different safety margins. The
fact that the advice to "replace all the capacitors" is often dispensed willy
nilly on the internet, presumably to be followed by people who are ignorant of
the implications, only exacerbates the problem. Though I have seen no academic
studies to back it up, common sense suggests to me that it is unhelpful advice,
and I stand by this.

Let's see some peer-reviewed, published studies on PCB repair policies and
practices - so at least we've got some facts - before flogging this one any
more, shall we?

I can't offer any peer-reviewed scientific studies to support the notion that
it is foolish to consider replacing all the nuts and bolts in a car to cure a
problem with it, or to replace all your roof tiles to cure one small leak, but
anyone with any experience of these things knows this is not necessarily the
best way to go about it.

Rod.
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"Roderick Stewart" wrote in
message .myzen.co.uk...

My assertion was concerned with swapping components unnecessarily without
doing
any diagnostics to establish which one is actually faulty. In my
experience,
any interference with things that are not faulty is a bad thing and can
lead to
further problems that were not originally present....

And in my experience it is often pragmatic to replace the components
associated with the "stock faults" as a matter of course.

I think where we do agree is that you ought to be a competent bench
technician to undertake repairs on modern PCBs, otherwise the risk of the
damage you describe is high.

But I happen to think that a competent bench technician should be able to do
most repairs without knock-on damage, and in particular to know when NOT to
do a repair because the risks are too high.

Really, though, we are only talking about opinions here. And I repeat: 45
years' experience is probably no better at all than 25 years', or even one
year's experience for a full-time bench tech.

SteveT

On Thu, 29 Apr 2010 21:12:58 +0100, Roderick Stewart
wrote:

I've just looked at an external SCSI DLT tape drive. That had bad caps
on the 12V output of the PSU, causing it to rise to 20V under load.
No overvoltage protection was fitted.

Can you explain how a faulty capacitor makes a voltage rise?

http://en.wikipedia.org/wiki/Equival...ies_resistance

In article en.co.uk,
Roderick Stewart writes

I have about 45 years of experience. Please enlighten me.

Hint: we don't use valves any more.

--
Mike Tomlinson

In article en.co.uk,
Roderick Stewart writes

Can you explain how a faulty capacitor makes a voltage rise?

I was under the impression you knew all about this.
Look up ESR and note my comment "no overvoltage protection was fitted".

For what it's worth, the PSU was made by Autec.

--
Mike Tomlinson

In article en.co.uk,
Roderick Stewart writes

My assertion was concerned with swapping components unnecessarily without doing
any diagnostics to establish which one is actually faulty.

As I have already said: does your man in the street have the diagnostic
equipment needed? We're not talking about a professional repair to a
highly integrated circuit board with surface-mount components, but
something that could potentially prevent Joe Bloggs tossing a good PVR
in the bin for want of a couple of quid's worth of capacitors and 15
minutes of prep and soldering. Only Joe will know if he is sufficiently
skilled to have a go and is willing to take the risk. If not, he may be
willing to find someone who will help him.

Your 45 years of experience certainly didn't teach you much in the way
of pragmatism or common sense. You've clearly not suffered from the
recent plague of bad caps or the desire of makers of electronic
equipment to build it down to the absolute minimum price, using the
cheapest components, but I assure you millions of others have.

--
Mike Tomlinson